The X-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical X-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor comprising a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the X-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
In an X-ray tube device with a rotatable anode, the target typically comprises a disk made of a refractory metal such as tungsten, and the X-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. High speed rotating anodes can reach 9,000 to 11,000 RPM. Rotation of the target is achieved by driving the rotor provided on a support shaft extending from the target.
Operating conditions for X-ray tubes have changed considerably in the last two decades. U.S. Pat. No. 4,119,261, issued Oct. 10, 1978, and U.S. Pat. No. 4,129,241, issued Dec. 12, 1978, were both devoted to joining rotating anodes made from molybdenum and molybdenum-tungsten alloys to stems made from columbium and its alloys. Continuing increases in applied energy during tube operation have led to a change in target composition to TZM or other molybdenum alloys, to increased target diameter and weight, as well as to the use of graphite as a heat sink in the back of the target. Future computerized tomography (CT) scanners will be capable of decreasing scan time from a one second rotation to a 0.5 second rotation or lower. However, such a decrease in scan time will quite possibly require a modification of the current CT anode design. The current CT anode design comprises two disks, one of a high head storage material such as graphite, and the second of a molybdenum alloy such as TZM. These two concentric disks are bonded together by means of a brazing process. A thin layer of refractory metal such as tungsten or tungsten alloy is deposited to form a focal track. Such a composite substrate structure may weigh in excess of 4 kg. With faster scanner rotation rates, heavy targets will increase not only mechanical stress on the bearing materials but also a focal spot sag motion causing image artifacts.
It would be desirable then to replace the present CT target design with a lightweight design comparable in thermal performance, particularly suited for use in X-ray rotating anode assemblies.